#include <bits/stdc++.h>

using namespace std;
using ll = long long;

int main(){
    int N;
    cin >> N;
    string s;
    cin >> s;
    ll ans = 0;
    int na = 0,nc = 0;
    for(int i = 0;i < N;++i){
        if(s[i] == 'A'){
            ++na;
        }else{
            ans += na;
            ++nc;
        }
    }
    ll best = ans;
    na = 0;
    // cout << ans << " " << na << " " << nc << endl;
    for(int i = 0;i < N;++i){
        if(s[i] == '?'){
            --nc;
            ans = ans - na + nc;
            // cout << ans << endl;
            best = max(best, ans);
            ++na;
        }else if(s[i] == 'A'){
            ++na;
        }else if(s[i] == 'C'){
            --nc;
        }
    }
    cout << best << endl;
    return 0;
}